At first, at the announcement of the games, I intended to get Ultra Moon, especially when I had the original Sun to begin with. However, as time passed and it became evident that USUM was a third version rather than a sequel like BW2, I started to deviate back to Ultra Sun, becoming fully prominent after seeing the exclusives for Ultra Sun and Moon. In addition, Dusk Mane Necrozma was better looking asthetic wise than Dawn Wings, which looked neat box art-wise, but in design wise, I felt Dawn Wings was ruined by how some of the armor was placed on it, especially the "tail" part which was just slapped onto Lunala's back. It was literally Lunala being sandwiched between the parts of Necrozma, and that didn't look very appealing in my eyes.
That and the box art of Ultra Sun, even though Dusk Mane didn't look as epic as Dawn Wings, was more appealing to me with the orange-red, black, yellow and gold colors used. More I owned Alpha Sapphire too, so I wanted a contrasting version to begin with, and Ultra Moon didn't give me that feeling.
In addition, I didn't play the original Sun much at all, nor finished it due to the existence of these games, so through this fact alone and from my personal opinions above, this finalized my decision to get Ultra Sun instead. By exchanging some old non-sellable games along with the original Sun at my local gamestore for store credit worth around 27 euros, I ultimately got my Ultra Sun copy for nothing less than 18 euros. I may get Ultra Moon later down the line maybe, not sure.